Coherent optical paired channel systems are known, for example in coherent optical Metro communication systems, in which a single laser can be used both for upstream transmission and as a local oscillator source for coherent downstream reception. FIG. 1 provides a schematic representation of the general principle and spectrum involved in such a system, generally referred to using the numeral 100. As shown in FIG. 1, the system generally includes a first tunable laser 102 for producing a first transmission or upstream signal 104 via a first transceiver 106, and a second tunable laser 108 for producing a second transmission or downstream signal 110 via a second transceiver 112. Each of the upstream and downstream signals will co-propagate on a same paired channel in opposite directions for respective coherent detection at the second and first transceivers 112 and 106, respectively. An optical fiber 114 of the system 100 is used bi-directionally; that is, the same fiber is used in both the downstream and upstream directions to propagate respective upstream 104 and downstream signals 110. As each optical transceiver 106, 112 consists of a transmitter and a receiver, the optical paths of both units have to be combined onto the single optical bi-directional fiber 114. While upstream and downstream separation on a single optical fiber has been accomplished using fixed band pass filters or optical circulators, each of these approaches generally suffers from one or more significant drawbacks.
For example, band pass filters used for this purpose were generally either realized as thin film filters or by doping glass such that the dopant causes the glass to absorb the light above a certain wavelength. Those realizations are not tunable and have a wide spectral transition region; the separation of two spectrally close wavelengths cannot be performed with such devices.
Circulators, on the other hand, can work for the separation of spectrally close or identical upstream and downstream wavelengths, but their integration into monolithic photonic circuits is not readily achievable at the time being due to fundamental physical reasons. Additionally, as circulators are spectrally wide-band, unwanted light which comes from back reflections from the fiber or back fiber connectors, for example, can pass through the circulator and ultimately cause distortions.
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